A vacuum degassing apparatus has conventionally been employed to remove bubbles generated in molten glass prior to shaping the molten glass in a shaping apparatus, the molten glass being produced by melting glass materials in a melting tank, so that the quality of shaped glass products can be improved.
The vacuum degassing apparatus comprises a vacuum degassing vessel the interior of which is maintained in a predetermined vacuum level. When the molten glass is passed through the vacuum degassing vessel, bubbles contained in the molten glass grow in a relatively short time. The grown bubbles rise in the molten glass owing to their buoyancy. When they reach the surface of the molten glass, they break themselves whereby the bubbles can be removed effectively from the surface of the molten glass.
From the viewpoints of increasing the amount of glass to be produced, reducing cost for producing glass etc., there has been a requirement for a large-sized equipment for producing glass, and also, for the vacuum degassing apparatus, there has been a requirement for increasing the ability of degassing, i.e. increasing the flow rate of the molten glass in the apparatus.
In order to increase the flow rate of the molten glass and to obtain a prescribed vacuum degassing treatment, it is necessary to consider fluctuations in various factors (for example, there are a fluctuation of the flow rate of the molten glass flow to which a degassing treatment is conducted, a fluctuation of the concentration of a gas component dissolved in the molten glass, which is caused by a temperature decrease of molten glass in the melting furnace, a fluctuation of the pressure in the depressurized vacuum degassing vessel, etc.) Under the consideration of these factors, it is necessary to bring the liquid surface of the molten glass into contact widely with an upper space in the vacuum degassing vessel whereby the bubbles generated in the molten glass can be removed to a prescribed range by degassing. In order to bring the liquid surface of the molten glass into contact widely with the upper space, the area of the bottom of the vacuum degassing vessel should be increased.
The area of the bottom of the vacuum degassing vessel can be increased by elongating the molten glass flow path in a longitudinal direction of the vacuum degassing vessel or by increasing the breadth of molten glass flow path. However, in a conventional vacuum degassing vessel having a long molten glass flow path, if the molten glass flow path is further elongated in its longitudinal direction, the thermal expansion in a longitudinal direction of the material for the flow path further increases whereby the flow path may be broken and the service life of the apparatus would be shortened. Further, such measures make the flow path longer as a whole, and therefore, the size of the apparatus whose vacuum degassing vessel be maintained in a predetermined degree of vacuum, is also increased. Accordingly, it is preferable to increase the breadth of molten glass flow path rather than the elongation of the molten glass flow path in its longitudinal direction. The inventors of the present application have proposed that the breadth of molten glass flow path is increased to increase the capacity of the apparatus, in Patent Document 1.
Patent Document 1 mentions the following two points as problems caused when the breadth of molten glass flow path of a vacuum degassing vessel is increased evenly.
First, it is anticipated that there takes place a molten glass flow of low flow rate in a local area at a downstream side of the molten glass flow path. In such case, the molten glass flow of low flow rate stays in that area longer than the molten glass flowing in the other area whereby light elements such as sodium (Na) evaporate during the stagnation so that the formulation of the molten glass in the local area may change. As a result, a final product such as a sheet-like glass has a local area having a different refractive index, which creates a distorted perspective image, whereby the ream deteriorates and the quality decreases.
Second, since it is difficult to produce a monolithic dense fire resistant brick which does not require any joint in a direction of the breadth of molten glass flow path e.g., a dense fire resistant brick having a breadth of 1 m, it is necessary to fabricate a molten glass flow path by assembling some dense fire resistant bricks in its breadth direction. Accordingly, it is unavoidable that joints exist in the ceiling, the bottom and both side walls which constitute the flow path in the vacuum degassing vessel. It is anticipated that among the above-mentioned joints, the joints at the connecting portions between the ceiling and the both side walls, and between the bottom and the both side walls spread easily owing to the thermal expansion of the fire resistant bricks when they are preheated for use or are heated for use. When the joints spread once, they are eroded severely by the molten glass and many bubbles generate from the spread joints, and these bubbles remain in the molten glass. Since these bubbles do not grow enough to be removed by degassing, the molten glass contains a large number of bubbles together with fine sand particles and is discharged from the vacuum degassing vessel. Thus, there is a problem of deterioration of the quality of glass products.
Patent Document 1 proposes the vacuum degassing apparatus to solve the above-mentioned first problem. The vacuum degassing apparatus for molten glass comprises a vacuum housing whose interior is vacuumed by air-sucking, a vacuum degassing vessel provided in the vacuum housing to degas the molten glass passing therethrough, an uprising pipe communicated with the vacuum degassing vessel to raise the molten glass by sucking before the degassing so as to introduce it into the vacuum degassing vessel and a downfalling pipe communicated with the vacuum degassing vessel to discharge downward the degassed molten glass from the vacuum degassing vessel, wherein the vacuum degassing vessel is so constituted that the inner breadth of flow path in a downstream portion in which the degassed molten glass falls through the downfalling pipe is narrower than the inner breadth of flow path in an upstream portion in which the molten glass is introduced through the uprising pipe.
Further, to solve the above-mentioned second problem, proposed is a vacuum degassing apparatus for molten glass which comprises a vacuum housing whose interior is vacuumed by air-sucking, a vacuum degassing vessel which is constituted by assembling some dense fire resistant bricks in the vacuum housing to degas the molten glass passing therethrough, an uprising pipe communicated with the vacuum degassing vessel to raise the molten glass by sucking before degassing so as to introduce it into the vacuum degassing vessel and a downfalling pipe communicated with the vacuum degassing vessel to discharge downward the degassed molten glass from the vacuum degassing vessel, wherein the vacuum degassing vessel comprises a ceiling, a bottom and both side walls which provide a flow path of rectangular shape in cross section by assembling some dense fire resistant bricks. The dense fire resistant bricks constituting the ceiling and the bottom which are connected with the both side walls have notched portions to which the dense fire resistant bricks of the both side walls fit. A fixing means is provided at the outside of the both side walls of the vacuum degassing vessel to fix the dense fire resistant bricks of the both side walls from the outside.